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UNIVERSITY    OF    ILLINOIS    LIBRARY    AT    URBANA-CHAMPAIGN 


L161— O-1096 


UNIVERSITY  OF  ILLINOIS 

Agricultural  Experiment  Station 


BULLETIN  No.  254 


ELIMINATION  OF  GERMS  FROM 
DAIRY  UTENSILS 

III.     STEAMING  CANS  OVER  A  JET 
BY  M.  J.  PBUCHA  AND  H.  A.  HARDING 


URBANA,  ILLINOIS,  AUGUST,  1924 


ELIMINATION  OF  GERMS  FROM 
DAIRY  UTENSILS 

III.     STEAMING  CANS  OVER  A  JET 

BY  M.  J.  PRUCHA,  CHIEF  IN  DAIRY  BACTERIOLOGY,  AND 
H.  A.  HARDING,  FORMERLY  CHIEF  IN  DAIRY  BACTERIOLOGY 

The  large  number  of  bacteria  commonly  found  in  market  milk 
has  long  been  a  matter  of  concern.1  The  bulk  of  the  raw  milk,  when 
it  reaches  the  milk  dealer,  contains  probably  more  than  100,000  bac- 
teria per  cubic  centimeter.  In  the  warmer  seasons  of  the  year  the 
number  is  larger,  some  millions  of  bacteria  per  cubic  centimeter  not 
uncommonly  being  found.  How  to  account  for  the  presence  of  these 
bacteria  in  the  milk  and  how  to  keep  them  out  of  the  milk  has  been 
a  subject  of  much  study. 

Previous  investigations  have  shown  that  probably  more  than  80 
percent  of  the  bacteria  in  fresh  market  milk  come  from  the  utensils 
in  which  the  milk  is  handled.2-  3,  4,  5,  e,  7  Among  the  utensils,  the  cans 
in  which  the  milk  is  shipped  to  the  milk  plant  are  of  outstanding 
importance.  The  milk  cans  are  the  source  of  this  surprisingly  large 
amount  of  germ  life,  not  necessarily  because  they  have  been  improp- 
erly washed  at  the  milk  plant,  but  because  they  have  not  been  prop- 
erly steamed  and  dried  after  the  washing  process.  By  the  time  the 
farmer  receives  such  cans,  some  10  to  40  hours  later,  the  bacteria  in 
them  have  increased  into  millions  and  they  are  unfit  to  receive  milk. 
While  it  is  possible  for  the  producer  to  put  such  cans  into  reasonably 
good  condition  on  the  farm,  it  is  manifestly  undesirable  that  cans  so 
heavily  laden  with  germ  life  should  be  returned  to  him. 

In  the  dairy  industry  the  steaming  of  cans  in  order  to  destroy 
the  germ  life  in  them  has  become  almost  universal.  Since  the  steam 
is  commonly  applied  to  the  cans  by  what  is  called  "jet  steaming," 
this  method  of  steam  application  is  the  subject  of  this  study. 

METHODS  OF  STUDY 

The  work  described  in  this  bulletin  included  the  bacteriological 
examination  of  1,348  milk  cans,  of  which  1,157  were  steamed  and 
191  were  left  unsteamed  as  check  cans.  These  cans  were  of  5-,  8-, 
and  10-gallon  capacity,  and  were  used  for  shipping  fresh  milk  from 
thirty-four  different  farms  to  two  milk  plants. 

The  amount  of  steam  which  is  blown  into  a  can  thru  a  jet  open- 
ing of  a  given  size  depends  upon  two  factors :  ( 1 )  the  length  of  time 
of  steaming,  and  (2)  the  pressure  at  which  the  steam  escapes  from 
the  jet  opening.  These  two  factors  were  varied  in  this  study.  The 

228 


BULLETIN  No.  254  229 

cans  were  steamed  for  3,  5,  10,  15,  20,  25,  30,  35,  40,  50,  60,  120, 
and  180  seconds,  and  the  pressures  used  were  3,  5,  10,  15,  20,  25,  30, 
35,  45,  and  50  pounds.  In  all,  thirty-six  different  combinations  of 
time  and  pressure  were  tested.  At  each  combination,  a  number  of 
cans  were  steamed  and  then  were  examined  for  bacteria.  A  jet  open- 
ing 14  inch  in  diameter  was  the  only  size  used. 

Washing  the  Cans. — The  cans  were  washed  in  two  different  milk 
plants  in  which  the  method  of  washing  varied  somewhat.  In  plant 
A,  about  50  gallons  of  water  was  used  to  wash  about  20  cans  and  the 
water  then  discarded.  After  being  washed,  the  cans  were  rinsed 
and  then  steamed.  In  plant  B,  only  about  25  gallons  of  water  was 
used  for  50  to  70  cans,  and  the  cans  were  not  rinsed  before  they  were 
steamed.  The  wash  water  in  both  piauts  contained  about  one  per- 
cent of  sodium-carbonate  washing  powder. 

Steaming  the  Cans. — The  cans  from  both  plants  were  steamed  at 
plant  A.  The  steam  used  was  conveyed  about  one-fourth  of  a  mile 
from  the  central  heating  plant  to  the  dairy  thru  a  large  pipe.  The 
steam  pressure  in  the  main  at  the  dairy  varied  from  70  to  90  pounds. 

The  equipment  for  jet  steaming  was  the  kind  commonly  found 
in  dairies.  A  strong  galvanized  iron  plate  20  inches  long  and  12 
inches  wide  was  fastened  horizontally  to  the  side  of  the  washing 
vat,  and  thru  the  center  of  this  plate  the  open  end  of  the  steam  pipe 
protruded  about  four  inches  above  the  surface.  The  can  to  be 
steamed  was  inverted  on  this  plate  and  the  steam  blown  into  it.  The 
covers  of  the  cans  were  steamed  separately  by  placing  them  in  a  metal 
box  and  inverting  the  box  over  the  steam  jet.  The  pressure  at  which 
the  steam  was  blown  into  the  cans  was  measured  by  a  sensitive  steam 
gage  placed  about  18  inches  from  the  jet  opening,  between  the  jet 
opening  and  the  valve  which  admitted  the  steam. 

Counting  the  Bacteria  in  the  Cans. — Each  can  to  be  examined 
was  rinsed  with  one  liter  (approximately  one  quart)  of  sterile  water, 
and  after  a  thoro  shaking,  so  as  to  bring  the  water  into  contact  with 
the  interior  of  the  can,  a  sample  of  the  water  was  taken.  The  germ 
count  of  this  water  was  determined  by  the  plate  method.  The 
total  number  of  bacteria  found  in  this  rinse  water  was  taken  as  the 
number  of  bacteria  to  have  been  in  the  can. 

The  medium  used  for  the  plating  was  an  agar  of  the  following 
composition:  agar  shreds,  15  grams;  Witte's  peptone,  10  grams; 
Liebig's  meat  extract,  3  grams;  lactose,  10  grams;  distilled  water 
to  make  1  liter.  The  reaction  of  this  medium  was  adjusted  to  one 
percent  normal  acid  to  phenphthalein.  The  plates  were  incubated 
seven  days;  five  days  at  20°  C.  and  two  days  at  37°  C. 

Amount  of  Steam  Blown  into  a  Can  per  Second  at  Different 
Pressures. — The  first  step  in  this  study  was  to  determine  the  amount 


230 


ELIMINATION  OF  GERMS  FROM  DAIRY  UTENSILS 


[August, 


of  steam  that  is  blown  into  the  can  at  the  different  pressures.  When 
steam  is  flowing  into  the  atmosphere,  the  amount  that  will  escape 
per  second  may  be  calculated  by  the  formula  EP,  provided  the  pres- 

~7o~ 

sure  at  which  the  steam  escapes  from  the  jet  opening  is  15  pounds 
or  more.8  In  this  formula,  E  stands  for  the  area  of  the  jet  opening; 
P  stands  for  the  absolute  pressure,  that  is  the  steam  pressure  plus 
the  atmospheric  pressure;  and  70  is  a  constant. 

When  the  steam  pressure  is  less  than  15  pounds,  this  formula  does 
not  hold.  Hence  it  was  necessary  to  make  direct  determinations  by 
the  following  method:  About  10  pounds  of  finely  crushed  ice  was 
placed  in  a  pail  and  the  steam  was  blown  into  it  for  a  given  length 
of  time  and  at  a  given  pressure.  The  pail  was  weighed  before  and 
again  after  the  steaming  and  the  difference  in  the  weight  gave  the 
amount  of  steam  in  pounds.  The  volume  of  steam  was  calculated 
on  the  basis  of  one  pound  equalling  26  cubic  feet. 

TABLE  1. — VOLUME  AND  WEIGHT  OF  STEAM  THAT  ESCAPES  PER  SECOND 
THRU  A  JET  OPENING  J4  INCH  IN  DIAMETER 


Steam 
pressure 

Amount 
of  steam 

Weight 
of  steam 

Steam 
pressure 

Amount 
of  steam 

Weight 
of  steam 

Ibs. 
3 
5 
10 
15 
20 
25 

cu.  ft. 
.1664 
.2704 
.4134 
.5177 
.6292 
.7280 

Ibs. 
0.0064 
0.0104 
0.0159 
0.0199 
0.0242 
0.0280 

/6s. 
30 
35 
40 
45 
50 

cu.  ft. 
.8190 
.9100 
1.0010 
1.0920 
1.1752 

Ibs. 
0.0315 
0.0350 
0.0385 
0.0420 
0.0452 

As  would  naturally  be  expected,  the  results  show  that  the  amount 
of  steam  blown  into  a  can  per  second  thru  a  given  opening  increases 
directly  with  the  pressure.  With  the  14-inch  opening,  at  3  pounds 
pressure  the  amount  is  .1664  cubic  feet  and  at  50  pounds  pressure 
it  is  1.1752  cubic  feet,  the  latter  being  more  than  seven  times  larger 
than  the  former.  It  is  evident  that  in  considering  the  efficiency  of 
steaming  as  a  means  of  destroying  germ  life  in  milk  cans,  the  pres- 
sure at  which  the  steam  is  blown  into  the  can  as  well  as  the  length 
of  time  should  be  taken  into  consideration. 


EXPERIMENTAL  RESULTS 

Bacterial  Count  of  the  Cans  Before  Steaming. — Information  re- 
garding the  bacteriological  condition  of  the  steamed  cans  just  before 
they  were  steamed  was  obtained  by  the  use  of  check  cans.  On  each 
day  after  the  cans  were  washed  and  were  ready  to  be  steamed,  a 
few  cans  were  selected  at  random  and,  without  being  steamed,  were 
examined  for  bacteria.  In  all,  191  such  check  cans,  constituting  14.2 
percent  of  all  the  cans,  were  thus  examined. 


1924} 


BULLETIN  No.  254 


231 


The  germ  counts  of  these  check  cans  varied  astonishingly,  but 
in  most  cases  the  count  was  large.  Of  these  191  cans,  2  appeared 
practically  free  from  germ  life;  2  others  showed  less  than  1,000,000 
bacteria;  in  110  cans  the  count  varied  between  one  million  and  one 
billion,  and  77  cans  had  a  count  of  more  than  one  billion  bacteria 
each.  The  largest  count  from  a  single  can  was  38,670,000,000.  The 
average  bacterial  count  per  can  was  over  2,357,000,000. 

The  effect  of  these  cans  upon  the  germ  count  of  milk  poured  into 
them  would  have  been  very  marked.  If  all  the  cans  had  been  filled 
with  milk,  the  average  increase  in  the  germ  count  of  the  milk  as  a 
direct  result  of  the  bacteria  in  the  cans  would  have  been  83,400  bac- 
teria per  cubic  centimeter. 

The  above  data  make  it  evident  that  the  cans  which  were  to  be 
steamed  were  abundantly  supplied  with  germ  life. 

Bacterial  Count  of  the  Steamed  Cans. — A  summary  of  the  results 
of  the  examination  of  the  1,157  steamed  cans  is  given  in  Table  2. 
The  data  are  so  arranged  as  to  show  the  effect  of  an  increasing  vol- 
ume of  steam.  As  the  volume  of  steam  applied  increased,  there  was 


TABLE  2. — RELATION  BETWEEN  AMOUNT  OP  STEAM  USED  AND  NUMBER 
OF  BACTERIA  LEFT  IN  STEAMED  CANS 


Time 
steamed 

Pressure 
of  steam 

Amount  of  steam 

Number 
of  cans 

Calculated  contamination  of 
milk  due  to  bacteria  in  cans 

Average  can 

Worst  can 

seconds 

Ibs. 

Ibs. 

c  u.  ft. 

germs  per  cc. 

germs  per  cc. 

3 

10 

0.048 

1.248 

21 

1,574 

22,048 

5 

5 

0.052 

1.352 

12 

44,838 

266,667 

5 

10 

0.079 

2.054 

24 

386 

2,408 

3 

25 

0.084 

2.184 

26 

1,166 

12,884 

10 

a 

0.104 

2.704 

9 

12,080 

45,333 

20 

3 

0.128 

3.328 

58 

11.923 

184.210 

5 

25 

0.140 

3.640 

69 

46,784 

1,666,667 

15 

5 

0.156 

4.056 

10 

5,962 

32,667 

10 

10 

0.159 

4.134 

65 

22,588 

473,684 

5 

35 

0.175 

4.550 

5 

246 

789 

30 

3 

0.192 

4.992 

55 

3,781 

52,368 

20 

5 

0.208 

5.408 

23 

603 

3,667 

5 

45 

0.210 

5.460 

38 

248 

3,777 

5 

50 

0.226 

5.876 

36 

100 

1,770 

40 

3 

0.256 

6.656 

23 

20 

227 

25 

5 

0.260 

6.760 

26 

452 

8,000 

10 

25 

0.280 

7.280 

29 

20 

447 

30 

5 

0.312 

8.112 

36 

269 

4,667 

10 

30 

0.315 

8.190 

38 

129 

5,789 

20 

10 

0.318 

8.268 

22 

779 

933 

50 

3 

0.320 

8.320 

30 

159 

2,362 

10 

35 

0.350 

9.100 

25 

21 

270 

15 

20 

0.363 

9.438 

25 

121 

1,867 

35 

5 

0.364 

9.464 

70 

1 

34 

60 

3 

0.384 

9.984 

35 

37 

449 

25 

10 

0.398 

10.348 

26 

24 

237 

20 

15 

0.399 

10.374 

24 

11 

138 

15 

25 

0.420 

10.920 

39 

8 

215 

30 

10 

0.477 

12.402 

73 

2 

45 

20 

20 

0.484 

12.584 

58 

6 

135 

25 

15 

0.498 

12.948 

51 

7 

152 

30 

20 

0.726 

18.876 

50 

2 

14 

30 

25 

0.840 

21.840 

6 

1 

1 

60 

25 

1.680 

43.680 

6 

1 

1 

120 

25 

3  .  360 

87.360 

6 

1 

1 

180 

25 

5.040 

131.040 

6 

1 

1 

0 

0 

0 

0 

191 

83,186 

1  ,289,000 

232  ELIMINATION  OF  GERMS  FROM  DAIRY  UTENSILS  [August. 

a  corresponding  decrease  in  the  number  of  bacteria  in  the  cans. 
Within  wide  limits,  it  apparently  is  not  very  important  whether  this 
volume  results  from  a  short  steaming  at  high  pressure  or  a  longer 
steaming  at  lower  pressure. 

The  volume  inclosed  by  a  8-gallon  can  is  practically  one  cubic 
foot.  When  the  steam  blown  into  the  can  was  equal  to  2  cubic  feet, 
the  destruction  of  germ  life  became  apparent.  When  5  cubic  feet 
of  steam  was  used,  the  average  number  of  bacteria  in  the  cans  after 
steaming  was  such  that  if  the  cans  had  been  filled  with  milk  the 
bacterial  count  of  the  milk  would  have  been  increased  less  than 
1,000  per  cc.  When  the  volume  of  steam  was  increased  to  about  9 
cubic  feet,  the  effect  of  the  can  on  the  milk  was  reduced  to  less  than 
100  bacteria  per  cc.  When  it  was  11  cubic  feet,  the  average  contamina- 
tion of  the  milk  by  the  can  was  reduced  to  less  than  10  per  cc. 

WHAT  IS  THE  BACTERIAL  COUNT  OF  A 
SATISFACTORY  CAN? 

It  is  evident  from  the  data  presented  that  it  is  impracticable  to 
obtain  sterile  cans  by  the  application  of  flowing  steam.  The  cans 
were  not  sterile  even  when  steam  at  a  pressure  of  25  pounds  was 
blown  into  them  continuously  for  three  minutes.  Since  steam  costs 
money,  it  is  desirable  to  have  in  mind  some  standard  as  to  the  maxi- 
mum number  of  bacteria  that  a  can  may  add  to  the  milk  and  still 
be  considered  satisfactory.  There  is  an  almost  complete  lack  of  in- 
formation or  even  discussion  regarding  this  point,  and  any  standards 
which  may  be  suggested  will  undoubtedly  be  found  inappropriate  un- 
der some  conditions. 

In  order  to  learn  the  opinions  of  those  best  qualified  to  judge,  a 
questionnaire  was  sent  to  250  individuals — representative  health  offi- 
cials, milk  dealers,  and  dairy  scientists.  The  questionnaire  explained 
the  need  of  a  definition  of  a  satisfactory  can,  and  asked  for  an  ex- 
pression of  judgment  as  to  the  maximum  number  of  bacteria  which 
such  a  can  might  add  to  the  milk  which  filled  it.  One  hundred  definite 
replies  were  returned,  and  these  are  summarized  below: 

Permissible  germs  per 
Number  of  replies  cubic  centimeter 

5 o 

i i 

i 5 

21 10 

1 25 

1 30 

6 50 

1 75 

58 100 

1 500 

2 1,000 

1 2,000   to   3,000 

1 10,000 


BULLETIN  No.  254 


233 


The  replies  represented  considerable  difference  of  opinion  but  the 
most  common  view  was  that  a  can  may  add  to  the  milk  put  into 
it  up  to  about  100  bacteria  per  cubic  centimeter  and  still  be  consid- 
ered in  satisfactory  condition. 

In  this  connection  it  should  be  kept  clearly  in  mind  that  putting 
a  can  into  satisfactory  condition  for  immediate  use  by  steaming  is 
an  entirely  different  matter  from  treating  it  so  that  it  will  be  in  a 
satisfactory  condition  ten  or  twenty  hours  later.  The  data  presented 
in  an  earlier  publication  of  this  Station6  show  that  if  steamed  cans 
are  allowed  to  remain  at  summer  temperature  and  moist,  the  growth 
of  germ  life  in  them  will  soon  render  them  unfit  for  receiving  milk. 


TIME  AND  STEAM  PRESSURE  REQUIRED  TO  PRODUCE 
A  SATISFACTORY  CAN 

It  is  evident  from  the  data  in  Table  2  that  the  application  of  at 
least  9  cubic  feet  of  steam  is  necessary  in  order  to  produce  a  can 
which  upon  being  promptly  filled  with  milk  will  not  increase  the 
germ  count  of  the  milk  more  than  100  bacteria  per  cubic  centimeter. 
In  translating  the  results  of  controlled  laboratory  experiments  into 
commercial  operations,  it  is  advisable  to  provide  for  a  margin  of 
safety.  This  margin  should  be  at  least  10  percent,  and  if  conditions 
permit,  it  should  be  about  25  percent.  Hence,  under  commercial 
conditions  the  amount  of  steam  per  can  should  be  increased  from  9 
to  about  12  cubic  feet. 

The  length  of  time  it  takes  to  blow  this  amount  of  steam  thru  a 
given  jet  opening  depends  upon  the  pressure  with  which  the  steam 
is  blown  thru  the  opening — the  higher  the  pressure,  the  shorter  the 
time.  At  3  pounds  pressure  it  would  take  72  seconds  to  force  12 
cubic  feet  of  steam  thru  a  jet  opening  14  inch  in  diameter,  while  at 
50  pounds  pressure  it  would  take  only  10  seconds. 

The  various  combinations  of  time  and  pressure  which  yielded 
approximately  9  to  12  cubic  feet  of  steam,  as  tested  in  this  study, 
are  given  in  Table  3.  In  considering  the  practical  application  of 
these  data,  it  should  be  remembered  that  where,  the  milk  is  brought 
to  the  bottling  plant  by  the  producer,  he  waits  at  the  receiving  plat- 

TABLE  3. — TIME  AND  PRESSURE  NECESSARY  TO  FORCE  9  TO  12  CUBIC  FEET 
OF  STEAM  THRU  A  JET  OPENING  %  INCH  IN  DIAMETER 


Time 

Pressure 

Steam 

seconds 

Ibs. 

cu.  ft. 

10 

35 

9.100 

15 

20 

9.438 

15 

25 

10.920 

20 

15 

10.374 

20 

20 

12.584 

25 

10 

10.348 

25 

15 

12.948 

30 

10 

12.402 

35 

5 

9.464 

60 

3 

9.984 

234  ELIMINATION  OF  GERMS  FROM  DAIRY  UTENSILS 

form  until  his  cans  are  ready  to  be  returned  to  him.  It  is  desirable 
that  the  washing  and  steaming  be  handled  as  quickly  as  possible 
in  order  to  cause  no  delay  in  the  receipt  of  the  next  load  of  milk. 
Where  the  milk  cans  are  shipped  by  train,  there  usually  is  an  equal 
necessity  for  haste  in  making  the  cans  ready  for  the  outgoing  trains. 
To  accomplish  the  work  in  the  time  available,  it  is  necessary  that  the 
minimum  amount  of  time  be  given  to  each  step  in  the  process. 

On  the  other  hand,  when  the  pressure  at  which  the  steam  is  forced 
thru  a  i^-inch  jet  opening  amounts  to  more  than  25  pounds,  there 
is  an  escape  of  an  undesirable  amount  of  steam  into  the  room.  This 
causes  inconvenience  to  the  workmen.  In  the  plants  where  cans  are 
dried  by  machines,  the  high  humidity  of  the  air,  which  is  partly  due 
to  this  escaping  steam,  interferes  with  proper  drying  of  the  cans. 
Moreover,  there  is  every  reason  to  believe  that  the  steam  which  thus 
escapes  does  not  exert  its  full  effect  upon  the  germ  life  in  the  cans. 

Considered  from  every  angle,  it  may  be  said  from  the  results  of 
these  experiments  that  the  steaming  of  cans  is  most  satisfactory  when 
the  flow  of  steam  is  so  controlled  that  it  requires  from  15  to  30  sec- 
onds to  blow  into  each  can  12  cubic  feet  of  steam. 

As  judged  by  repeated  observations,  the  cans  in  commercial  dairy 
plants  are  not  as  a  rule  steamed  uniformly  nor  are  they  steamed 
sufficiently.  This  is  due,  in  part  at  least,  to  the  fact  that  the  steam- 
ing is  subordinated  to  other  plant  operations.  The  length  of  time 
the  can  is  steamed  is  set  by  the  length  of  time  it  takes  the  operator 
to  wash  a  can  or  to  empty  a  can ;  such  steaming  is  not  reliable.  The 
length  of  steaming  should  be  determined  by  the  steam  pressure  and 
the  size  of  the  jet  opening,  and  may  be  readily  calculated  by  using 
the  formula  given  on  the  preceding  page. 

If  the  steaming  of  cans  in  the  commercial  dairy  plants  were  prop- 
erly standardized,  a  marked  improvement  in  the  bacteriological  con- 
dition of  the  milk  supply  would  result. 

LITERATURE   CITED 

1.  Milk  and  Its  Relation  to  the  Public  Health.     Public  Health  and  Marine-Hospital 
Service.      Hygienic   Laboratory,   Bui.   56.      1909. 

2.  Prucha,    M.    J.,  W-eeter,   H.    M.,   and   Chambers,   W.    H.      Germ   Content   of   Milk: 
II   As  Influenced  by  the  Utensils.      111.   Agr.  Exp.   Sta.  Bui.   204.      1918. 

3.  Ayers,    S.    Henry,    Cook,    Lee   B.,    and    Clemmer,    Paul  W.      The    Four    Essential 
Factors  in  tho  Production  of  Milk  with  Low  Bacterial  Content.     U.  S.  D.  A.  Bui.  642.  1918. 

4.  Conn,   W.   H.      Practical  Dairy   Bacteriology.      1910. 

5.  Russell,  H.   L.     The   Source  of  Bacterial  Infection  and  the  Relation   of  the   Same 
to  the  Keeping  Quality  of  Milk.     Wis.  Agr.  Exp.   Sta.  Ann.  Rpt.   11,   pp.   151-152.      1894. 

6.  Bergey,  D.  H.      Sources  and  Nature  of  Bacteria  in  Milk.     Penn.  Agr.  Dept.   Bui. 
125.      1904. 

7.  Prucha,  M.   J.,  and  Harding,   H.   A.      Elimination   of  Germs   from   Dairy  Utensils: 
I  by  Rinsing;    II  by  Drying  in  Sun  and  Air.     111.  Agr.  Exp.   Sta.  Bui.  230.      1920. 

8.  Kent,   William.      The    Mechanical    Engineer's   Pocket   Book,   p.    844.      1912. 

ACKNOWLEDGMENT 

The  data  reported  in  this  bulletin  were  obtained  during  1915,  1916,  and  1917. 
Messrs.  H.  M.  Weeter  and  W.  H.  Chambers,  then  members  of  the  Dairy  Department, 
took  an  active  part  both  in  the  routine  conduct  of  the  experiment  and  in  developing 
the  plans  for  the  study.  Their  faithful  service  and  intelligent  interest  are  gratefully 
acknowledged. 


UNIVERSITY  OF  ILLINOIS-URBANA 


